1. Field of the Invention
This invention relates to an improved process for producing leather and products resulting therefrom. These compositions are copolymers of methacrylic acid and one or more alkyl acrylates and are characterized by a narrow molecular weight range. These tanning compositions and processes are useful in the pretanning, tanning, and retanning of leather. The leather produced by this improved process is flexible, with a smooth grain and well-filled, and is resistant to detannage by washing with alkaline solution.
2. Brief Description of the Prior Art
Phenolic syntans are synthetic resin tanning agents which can be used as sole tanning agents to replace natural tanning agents or in combination tannages to control leather fullness, yield, color, and processing properties. The use of phenolic syntans, however, presents a problem in that the effluent stream from the tanning may contain phenol and formaldehyde, two toxic materials. Thus there is a need for a synthetic tanning agent which can replace phenolic syntans in the tanning process without engendering toxic waste by-products. The synthetic methacrylic acid-alkyl acrylate copolymer compositions of the present invention can be used as sole tanning agents or in combination tannages with vegetable tannin, mineral or chrome tanning agents. These compositions are useful replacements for phenolic syntans. These compositions do not engender phenol or formaldehyde containing effluents. In addition, they exhibit light and oxidation stability which are superior to that shown by the phenolic syntans.
The use of acrylic acid and methacrylic acid copolymers as tanning compositions is well known in the art. For example, Graves in U.S. Pat. No. 2,205,882, granted June 25, 1940, discloses a process for tanning leather which comprises subjecting a skin to the action of an acidic polymeric material in which the acidity is due to carboxyl groups attached to aliphatic carbon atoms. Polyacrylic acid and copolymers of acrylic acid and methacrylic acid, and maleic anhydride and styrene are also disclosed to be useful. In U.S. Pat. No. 2,205,883 granted June 25, 1940, Graves discloses an improved process for tanning leather employing acid polymerization products of methacrylic acid as tanning agents. Polymethacrylic acid, partially hydrolyzed methyl methacrylate, methacrylic acid-styrene copolymer and methacrylic acid-methyl methacrylate copolymer are taught. Graves further discloses that solution viscosity and molecular weight may affect the rate and degree of tanning somewhat and further warns that very low molecular weight polymers should be avoided (Column 8, lines 56-66).
The methacrylic acid/alkyl acrylate copolymers of the instant invention are characterized by a specific and narrow molecular weight range. Surprisingly, this narrow molecular weight range is essential to realizing effective tanning of the leather stock. Also, the selection of methacrylic acid over acrylic acid is surprisingly essential as is the use of short chain alcohol esters of acrylic acid as comonomers with the methacrylic acid to achieve substantial resistance to detannage and grain cracking of the leather produced by the process.
An improved tanning composition made by copolymerization of a nixture of (a) methacrylic acid, acrylic acid, or mixtures of such acid with (b) a sulfated unsaturated oil is disclosed by W. J. Rau in U.S. Pat. No. 3,408,319, granted Oct. 29, 1968. Rau discloses that the most suitable molecular weight range of the copolymer product is from about 5000 to 50,000 number average (Column 2, lines 58-60).
J. A. Lowell et al. in U.S. Pat. No. 3,231,420, granted Jan. 25, 1956, disclose a process for providing leathers having improved break, fuller substance and improved resistance to abrasion and scuffing, which process comprises impregnating leathers with organic solvent solutions of copolymers and plasticizers. The water insoluble copolymers employed are prepared from (a) an acid selected from the group consisting of acrylic, methacrylic and itaconic acids, (b) at least one ester of acrylic or methacrylic acid and a saturated monohydric aliphatic alcohol having between 8 and 18 carbons atoms, (c) methyl, ethyl, or isobutyl methacrylate and (d) an ester of acrylic acid with a saturated monohydric alcohol having 1 to 4 carbon atoms. Lowell teaches that each of the 4 types of monomers employed in the polymer is essential. Further, in order to obtain good qualities in the impregnated leather, the average molecular weight of the copolymer is taught to be at least 10,000 (Column 2, lines 58-60).
A process for the filling of leather in compositions employed therein is taught by Wurmli et al. in U.S. Pat. No. 3,945,792, granted Mar. 23, 1976. The filling agent employed may consist of a homopolymer or copolymer selected from acrylic or methacrylic acid, or copolymers formed from acrylic or methacrylic acid and a vinyl unsaturated monomer selected from water insoluble comonomers such as acrylic or methacrylic acid alkyl esters having from 1 to 12 carbon atoms in the alkyl radical and a protein glue in a weight ratio of polymer to glue of from 1:12 to 12:1.
W. C. Beier et al. describe in U.S. Pat. No. 4,312,802, granted Feb. 9, 1982, a multistage process for producing leather, one step of which consists of treating leather stock with a tanning composition comprising an aqueous dispersion or solution of a polymer polymerized from a monomer mixture comprising at least one member selected from the group consisting of acrylic acid, methacrylic acid, mixtures of acrylic acid and methacrylic acid and mixtures of a major proportion of at least one member selected from the group consisting of acrylic acid and methacrylic acid with a minor proportion of at least one component selected from the group consisting of alkyl esters of acrylic acid, alkyl esters of methacrylic acid and partially sulfated unsaturated drying oils. Beier et al. do not disclose any limitation on the molecular weight of methacrylic acid alkyl acrylate copolymer tanning agents. Beier et al.'s process further requires an additional tanning step. The copolymers of the instant invention may be used to tan leather for shoe soles.
R. Monsheimer et al. disclose in U.S. Pat. No. 4,314,800, granted Feb. 9, 1982, a method for treating pelts and leather in the beam house for tanning or as an after treatment which consists of contacting such pelts or leather with an aqueous solution or dispersion of a copolymer comprising monomers of a nitrogenous ester of acrylic acid or methacrylic acid, a further different ester of acrylic acid or methacrylic acid and acrylic acid or methacrylic acid whereby portions of the copolymer are deposited in the grain boundary without formation of a superficial film.
H. Lochel disclose in U.S. Pat. No. 4,345,006, granted Aug. 17, 1982, methods for treating tanned leather with a dispersion of an acrylate resin comprising certain acrylate and/or methacrylate esters, a hydroxyalkyl acrylate or methacrylate, an unsaturated polymerizable anionic compound such as an unsaturated carboxylic acid, a crosslinking monomer and, optionally, acrylamide or methacrylamide.
In situ polymerization of aqueous dispersions of monomers and polymers in order to incorporate large proportions of high molecular weight polymers in leather in order to improve leather mechanical properties is taught by E. Bader et al. in U.S. Pat. No. 3,291,558, granted Dec. 13, 1966. Similarly, sheepskin leather which exhibits an unusual weak internal fiber strength tending to cause "double hiding" can be remedied by precipitating an elastomeric resin material within the leather by repeatedly subjecting the leather to pressure and release of the pressure according to R. H. Doggett et al. as disclosed in U.S. Pat. No. 3,245,832, granted Apr. 12, 1966.
None of the prior art references disclose or suggest the specific and narrow molecular weight range methacrylic acid-short chain alkyl acrylate copolymer tanning agents of the present invention.